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Fission track (FT) ages of apatite and zircon from four grantie batholiths from Lhasa and Shannan areas are measured. The FT ages of apatite range from 3.2 to 8.3 Ma, corresponding to the uplift rates of 0.12 to 0.20 mm·a[SUP-1] during this period. The uplift height is 580 m, showing that there is not large-scale rapid uplifting in southern Tibet from 3.2 to 8.3 Ma. The zircon FT ages of Lhasa batholith are 25.9 ± 1.7 and 32.7 ± 2.8 Ma, yielding an uplift rate of 0.08 mm·a[SUP-1] between 26 and 33 Ma. Combining this work with other studies, it is suggested that the average uplift rate in southern Tibet is low from the time of collision between India and Asian continents to -3 Ma. The uplift of Tibetan Plateau seems to have finished in multi-stage processes with varied rates. fission track age, uplift rate, Lhasa, Tibet [ABSTRACT FROM AUTHOR]

Examines the electrical conductivity of dunite at high temperature and pressure in the Qinghai-Xizang Plateau in Tibet, China. Determination of dunite activation enthalpies; Factors contributing to the conduction mechanism in dunite; Assessment on the crust-mantle thermal structure in the plateau.

The lithium isotopic compositions of adakitic rocks and K-rich volcanic rocks in southern Tibet range from + 1.3‰ to + 7.5‰ and − 4.9‰ to + 3.5‰, respectively. The subduction of the Indian Plate beneath the Lhasa Terrane means that traditional lithium isotopic compositions of various reservoirs are unable to explain the aforementioned δ 7 Li data. Therefore, it is necessary to determine the Li isotopic compositions of the different geological endmembers underneath southern Tibet. Here, we report the lithium isotopic composition and concentration of Indian upper and lower continental crust. On the basis of whole-rock major and trace element data and Sr–Nd–Pb isotope data, leucogranites from Luozha and Longzi are considered as representative of Indian upper crust, whereas two-mica granites from Quedang and Dala and granulites and gneisses from Nyalam are considered as representative of Indian lower crust. The Li concentration of Indian upper crust varies from 23 to 45 ppm with a mean of 34 ppm, consistent with a weighted mean Li concentration for the upper crust of 35 ± 11 ppm. In contrast, the Li abundance of Indian lower crust is estimated to be 33–84 ppm with a mean of 58 ppm, much higher than the average Li concentration for the lower crust of ~ 8 ppm because of the high modal abundance of biotite and muscovite. The Li isotopic compositions (+ 0.9‰ to + 5.6‰) of Indian upper continental crust are relatively heavy compared with the average Li isotopic composition of upper continental crust. On the other hand, the Li isotopic compositions of Indian lower continental crust (− 4.4‰ to − 0.1‰) are lighter than those of Indian upper continental crust. The isotopically heavy signature of Indian upper crust is produced by high-δ 7 Li fluids released from the Indian lower crust slab, whereas the lighter signature of Indian lower crust is generated by the partial melting of residual Indian lower crust slab during metamorphic dehydration of Indian lower crust. [ABSTRACT FROM AUTHOR]

Understanding the geological history of the Lhasa Terrane prior to the India–Asia collision (~ 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene (~ 18–10 Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183–174 Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I–S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane. [ABSTRACT FROM AUTHOR]

The Neo-Tethys oceanic slab rollback and ridge subduction are two competing geodynamic processes responsible for the early stage of late Cretaceous magmatic activity. The spatial and temporal distribution of geochemical variability in igneous rocks can be used for reconstructing specific geodynamic events. We presented the whole-rock geochemistry, zircon U Pb ages, mineral chemistry data of the Gongga adakitic host rocks and coeval dioritic enclaves emplaced in the southern Lhasa terrane. The Gongga adakitic host rocks have low Mg# (43.7– 44.9), but high in K 2 O/Na 2 O > 0.6, indicating they may be derived from the juvenile lower crust. According to geochemical modeling, the adakitic host rocks were formed by about 40% partial melting of the metamorphic gabbroic diorite with adakitic affinity. The typical magmatic texture and reverse oscillatory zoning of plagioclase in dioritic enclaves imply that they may be generated during the process of magma mixing. By the integrated analysis of new geochemical data of the Gongga intrusive rocks and compiled spatio-temporal distribution and geochemical data of Cretaceous magmatism in the southern Lhasa terrane, we proposed that the initial Neo-Tethyan oceanic slab rollback occurred at ∼98 Ma, causing the production of the Gongga host rocks and magmatic encalves. The Neo-Tethys ridge subduction and slab rollback jointly took part in the production of the magmatic flare-up during the late Cretaceous. • The Gongga adakitic host rocks were formed from the juvenile lower crust. • The Gongga dioritic enclaves were produced by magma mixing. • The Gongga pluton were produced during the initial slab rollback of Neo- Tethys Ocean. • The Neo-Tethys slab rollback and ridge subduction together caused magmatic flare-up. [ABSTRACT FROM AUTHOR]

Magnesium isotopic compositions of igneous rocks could be potentially used to trace recycling of supracrustal materials. High-δ 26 Mg granitoids have been previously reported and explained to reflect the involvement of surface weathered materials in their sources. Low-δ 26 Mg granitoids, however, have not been reported. In this study, we report high-precision Mg isotopic analyses of Cenozoic alkaline syenites and syenogranites from the Kuzigan and Zankan plutons, northwest Xinjiang, China. The Kuzigan syenites were originated from the mantle metasomatized by recycled supracrustal materials, and the syenogranites are differentiated products of the syenites. Both syenites and syenogranites have δ 26 Mg values (− 0.46 to − 0.26‰ and − 0.41 to − 0.17‰, respectively) significantly lighter than the mantle (− 0.25 ± 0.07‰, 2SD). No correlation of δ 26 Mg with either SiO 2 or MgO is observed, indicating limited Mg isotope fractionation during alkaline magmatic differentiation. The low δ 26 Mg of the syenites and syenogranites thus reflects a light Mg isotopic source. This, combined with high 87 Sr/ 86 Sr ratios (0.70814 to 0.71105) and negative correlation between δ 26 Mg and δ 18 O, suggests that the magma source contains recycled marine carbonates. Modeling of the Mg-O-Sr isotopic data indicates that the recycled carbonate is mainly limestone with minor dolostone, suggesting that the metasomatism occurred at depths shallower than 60 to 120 km. Given that the plutons are located at the India–Eurasia collision zone, the carbonate recycling was most likely derived from the subducted Tethyan oceanic crust during the Mesozoic–Cenozoic. Our study suggests that the combined Mg, O, and Sr isotopic studies are powerful for tracing recycled carbonates and identifying their species in mantle sources. [ABSTRACT FROM AUTHOR]

The history of crustal thickening is critical for comprehending the evolution process of the Tibetan plateau. The genetic relationship and spatiotemporal distribution of adakitic rocks with high Sr/Y, (La/Yb) N ratios may impose fundamental constraints on specific geodynamic and crustal thickening processes. The Songka intrusive suite consists of adakitic rocks with high-Mg adakite (monzonites), low-Mg adakitic rocks (quartz monzonites), and gabbroic-diorites. Their geochemical characteristics, petrogenesis, and geodynamic implication is discussed in this paper. Songka high-Mg adakites formed when subducting Neo-Tethyan oceanic slab melts were assimilated by mantle peridotite at elevated temperatures (average Ti-in-zircon temperatures of sample SK1319 are 830 °C). Low-Mg adakitic rocks exhibit markedly different geochemical characteristics than those formed in the juvenile thickened lower crust. The least-squares mass balance and trace element simulations indicate that Songka low-Mg adakitic samples were formed at a degree of 60–65% fractional crystallization of melts resembling Songka high-Mg adakites. Songka gabbroic diorites are formed in the metasomatic mantle wedge. The genetic relationship between these rocks indicates that the crust of the southern Tibet did not appear to thicken obviously during the early stage of late Cretaceous (100–90 Ma). The intensive emplacement of late Cretaceous high-Mg adakites (Mg# > 50) and mafic rocks (SiO 2 < 52%) in the southern Lhasa terrane around 95–90 Ma, together with the slow convergence velocity of the Indian-Asian continent, can be explained by the response of rapid crustal thickening and Neo-Tethyan slab rollback at 95–90 Ma. • High-Mg adakitic rocks were formed from partial melting of Neo-Tethyan slab. • Coeval low-Mg adakitic rocks were produced by fractional crystallization. • Rapid crustal growth and Neo-Tethyan slab rollback occurred at 95–90 Ma. [ABSTRACT FROM AUTHOR]

Abstract: Early Triassic syn-collisional granitoids with mafic magmatic enclaves (MMEs) crop out along the entire East Kunlun Orogenic belt (EKOB) at the northern margin of the Tibetan Plateau. They are andesitic in composition and enriched in light rare earth elements (LREEs) with a flat heavy REE (HREE) pattern. Their average composition resembles that of the bulk continental crust. The enclosed MMEs have the same mineralogy as their host granitoids, but contain a greater mode of mafic minerals (amphibole and biotite), and thus have higher HREE abundances. Zircon U–Pb dating shows that both the granitoid hosts and MMEs have the same crystallization age of ~250Ma and indistinguishable bulk rock Sr–Nd–Pb–Hf isotope compositions (ISr of 0.7080–0.7116, varying 206Pb/204Pbi of 18.53–19.32, essentially constant εNd(t) of −5.3 to −2.1 and a small range of positive εHf(t), mostly 1.7–5.2). The complete isotopic overlapping between the granitoid hosts and the MMEs is understood to reflect that the MMEs are disintegrated cumulates formed at an early stage of the granitoid magma evolution within the same magmatic system. The isotopic data set reveals that the granitoids are variably evolved melts produced by partial melting of the subducted Paleo-Tethyan oceanic crust with terrigenous sediments under amphibolite-facies conditions in response to the continental collision. [Copyright &y& Elsevier]

Abstract: The significance of the continental crust (CC) on which we live is self-evident. However, our knowledge remains limited on its origin, its way and rate of growth, and how it has acquired the “andesitic” composition from mantle derived magmas. Compared to rocks formed from mantle derived magmas in all geological environments, volcanic arc rocks associated with seafloor subduction share some common features with the CC; both are relatively depleted in “fluid-insoluble” elements (e.g., Nb, Ta and Ti), but enriched in “fluid-soluble” elements (e.g., U, K and Pb). These chemical characteristics are referred to as the “arc-like signature”, and point to a possible link between subduction-zone magmatism and CC formation, thus leading to the “island arc” model widely accepted for the origin of the CC over the past 45years. However, this “island–arc” model has many difficulties: e.g., (1) the bulk arc crust (AC) is basaltic whereas the bulk CC is andesitic; (2) the AC has variably large Sr excess whereas the CC is weakly Sr deficient; and (3) AC production is mass-balanced by subduction erosion and sediment recycling, thus contributing no net mass to the CC growth, at least in the Phanerozoic. Our recent and ongoing studies on granitoid rocks (both volcanic and intrusive) formed in response to the India–Asia continental collision (~55±10Ma) show remarkable compositional similarity to the bulk CC with the typical “arc-like signature”. Also, these syncollisional granitoid rocks exhibit strong mantle isotopic signatures, meaning that they were recently derived from a mantle source. The petrology and geochemistry of these syncollisional granitoid rocks are most consistent with an origin via partial melting of the upper ocean crust (i.e., last fragments of underthrusting ocean crust upon collision) under amphibolite facies conditions, adding net mantle-derived materials to form juvenile CC mass. This leads to the logical and testable hypothesis that continental collision produces and preserves the juvenile crust, and hence maintains net CC growth. Importantly, the history of the Greater Tibetan Plateau from the Early Paleozoic to present manifests the history of “super” continent amalgamation through a series of continental collision events with production and preservation of abundant syncollisional granitoids. Plate tectonics in terms of seafloor spreading and subduction is a continuous process on a global scale since its inception (in the early Archean?), whereas continental collision on regional scales and super-continental formation on a global scale are episodic (vs. continuous). Hence, continental collision with juvenile crust formation/preservation and super-continent amalgamation explains the episodic growth of the CC. We are continuing testing and refining this hypothesis by detailed petrological, geochemical and geochronological studies of syncollisional granitoids along older collision zones in central-west China, especially on the northern Tibetan Plateau in a global context. [Copyright &y& Elsevier]

Post-collisional potassic and high Sr/Y magmatism in the Lhasa terrane provides critical constraints on the timing and mechanism of subduction of Indian lithosphere and its role in the uplift of the Tibetan Plateau. Here, we report whole-rock geochemistry, mineral geochemistry, zircon U Pb ages, and in situ zircon Hf isotope ratios for the Nyemo pluton, a representative example of such magmatism. The Nyemo pluton is composed of high Sr/Y host rocks and coeval shoshonitic mafic microgranular enclaves (MMEs). Whole-rock compositions of the host rocks and MMEs form linear trends in Harker diagrams, consistent with modification of both end-members by magma mixing. Although the main high Sr/Y phase of the pluton formed by partial melting of the lower crust of the thickened Lhasa terrane, the MMEs display abnormally enriched light rare earth elements, low whole-rock ε Nd (t) and low zircon ε Hf (t) that suggest derivation from low degree melting of hydrous and enriched mantle. Based on the occurrence of shoshonitic magma and high La/Yb and high Sr/Y with adakitic affinity host rocks around 30 Ma, the Nyemo pluton is best explained as a record of onset of extension that resulted from convective removal of the mantle lithosphere beneath Tibet in the Oligocene. • High Sr/Y host rocks of Nyemo pluton are derived from thickened lower crust. • Coeval shoshonitic enclaves were produced from enriched lithospheric mantle. • Lithospheric mantle convective removal, onsets of extension may have occurred during Oligocene. [ABSTRACT FROM AUTHOR]